A method for continuous production of high-molecular polybutylene terephthalate by esterification of terephthalic acid with 1,4-butanediol is known from German Patent 35 44 551. In this process, starting materials are blended to form a paste and sent for esterification in the presence of a suitable catalyst. Esterification is performed at pressures of 0.1 bar to 0.6 bar at which water can also be split off. In the main reaction of the carboxyl groups with the hydroxyl groups of 1,4-butanediol, dehydration of the butanediol also takes place, leading to tetrahydrofuran, but other byproducts whose structure and quantity depend on the 1,4-butanediol synthesis process may also be formed in low concentrations.
The polycondensation which follows esterification is preferably performed at pressures of 0.25 to 25 mbar and temperatures of approximately 270° C., but it is only possible to work in a single reaction apparatus from the group of stirred vessels, horizontal units of the ring disk type and/or cage reactor type. A resulting long chain polycondensate contains the monomer building blocks in a random distribution.
The copolyesters produced by the methods known so far have valuable elastomeric properties and are therefore being used commercially to an increasing extent. Consequently there is a demand for further improvements in and simplifications of the known processes, in particular in reducing the so far substantial cost of equipment and utilities.